Beer, wine and spirituous liquor, as well as other lesser known alcoholic beverages, are the fermentation products of yeasts such as Saccharomyces cerevisiae or in the case of beers, usually S. carlsburgiensis. Yeasts are unicellular fungi that reproduce asexually by budding or fission and the reaction by which alcoholic beverages are produced is generally referred to as “fermentation” and may be summarized as: Yeast+Glucose→Alcohol (Ethanol)+CO2, wherein the production of alcohol occurs best in the absence of oxygen.
For the yeast, both the alcohol and carbon dioxide are waste products, and as the yeast continues to grow and metabolize in the sugar solution the accumulation of alcohol becomes increasingly toxic such that when the yeast is exposed to a high concentration of yeast (e.g. 12-18% v/v) for an extended period of time the yeast cells are lysed and ultimately killed. The extended exposure of yeast to such high concentrations of ethanol reduces the yeast extract yield and the yeast recovered is usually only recovered for low value ruminant animal feed.
The alcohol concentration in beer is considerably lower than wine (9-15% v/v) and is typically about 5% v/v. Once the beer has reached its optimum alcohol level the yeast can be recycled and used again until it eventually dies or, alternatively, small amounts of yeast can be recovered if they are in relatively good condition. To be useful as a food grade product, the recovered yeast is usually de-bittered to remove hop-derived flavours.
In the past 50 years increasing attention has been focussed on production of fuel grade ethanol. The emphasis in production of fuel ethanol is to maximise efficiency of ethanol production. This is normally achieved by providing the minimum amount of yeast necessary to effect the efficient and rapid conversion of sugar to ethanol. Often the yeast is partially recycled from the end of the fermentation process so that it can be re-used for as long as possible. The yeast at the end of the process is of poor nutritional value, and is normally recovered together with any non-fermentable material in the medium, and used for ruminant animal feed.
The production of yeast for human food consumption, such as for example, in the baking industry, or for further processing to a range of products used as flavours or flavour enhancers, involves aerobic fermentation. Yeast for such purposes is most efficiently produced with minimum production of ethanol or other by-products.